Electro-Absorption modulators are commonly used in many modern communication components for data transmission. They are often integrated with lasers, which are of a nominally fixed wavelength for use in WDM or CWDM applications. This makes them highly temperature sensitive requiring them to have excellent temperature control. This translates to high cost and high power consumption per bit thus effectively eliminating their ability to be used in low cost LAN devices or for inside the box applications or other short haul applications. In addition, they tend to be long with a large area making them hard to drive at high speeds.
Direct modulation of VCSELs has reached its limit for speed in LAN applications due to fundamental device physics at a bandwidth of around 25 GHz. To solve this problem VCSELs with integrated modulators have been proposed, but none has been able to demonstrate suitable performance. The dominant problem is the nearly fixed wavelength of the VCSEL combined with the large change in the absorption spectrum of the modulator with temperature.
To solve this problem a laser whose wavelength tracks the absorption spectrum of the modulator as a function of temperature can be used. Combining this with a modulator with a wide spectral response allows a large temperature range with substantial process tolerance. Also by using a modulator with a high absorption coefficient using direct band to band quantum well absorption the size and thus the capacitance of the modulator can be reduced significantly enhancing the speed.
Another problem is the variation that arises with the modulation of back reflection from the modulator section into the laser. This results in the modulation of the laser and resultant eye closure. One method of dealing with this problem is through the use of an optical isolator. However, this adds a great deal of extra cost and complexity and is not practical for a low cost integrated modulator.
One solution for minimizing back reflection variation is by using an output coupler after the modulator, which has very high output coupling and low back reflection. There are various structures that have been proposed that allow high output coupling with low back reflection.
One example of a possible structure is simply to provide for an anti-reflection coating at the output end of the modulator. This has the problem that external components could reflect back into the laser if not intentionally aligned to avoid this. Intentionally aligning to reduce back reflection is a possible solution but the downsides are: high cost, difficulty of facet coatings and horizontal emission, which is inconvenient for coupling to fibers.
Another example of a possible structure is to provide for a micro-machined output mirror at or near 45 degrees at the output end of the modulator. This is a suitable method to achieve low back reflection. With AR coatings it can be used effectively to achieve the desired goals of low back reflection and nominally vertical coupling, which can be designed to be somewhat off vertical. It has one primary difficulty. The tight vertical mode confinement in the modulator and laser section required for optimal performance translates to an excessively high beam angle. This can be accounted for with suitable lensing on the backside of the device if substrate emission is used or other external optics.
A more preferable method to achieve high output coupling is to provide for a high efficiency grating coupler. This can be designed to avoid coupling back reflected light into the modulator and can be used to achieve (near) vertical emission from the surface or through the substrate. The high efficiency grating coupler can launch the light off vertical to avoid coupling back into the laser from downstream optics and is effective at coupling the highly confined modes to a low divergence beam.
For the foregoing reasons, there is a need for temperature insensitive electro-absorption modulator and laser. The laser has a wavelength that tracks the absorption spectrum of the electro-absorption modulator (“EAM” or “modulator”) as a function of temperature. This mutually tracking laser-modulator allows a large temperature range with substantial process tolerance. Also, the modulator has a high absorption coefficient using direct band to band quantum well absorption, thereby significantly reducing the size and thus the capacitance of the modulator, bringing about significant enhancements in speed.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one example technology where some embodiments described herein may be practiced.